The present invention relates broadly to a method and an apparatus for accurately reproducing pulse width modulated (PWM) signals represented on magnetic tape. More specifically, the present invention relates to the magnetic recording and reproduction of physiological information, such as in electro-cardiograms, in PWM format.
Pulse width modulation is sometimes utilized in magnetic tape recording wherein the range of frequencies to be recorded has only a moderate passband, for example from DC to some low frequency. Recording in PWM format is advantageous because playback can be accomplished with substantial fidelity. However, PWM techniques have some serious disadvantages when compared with frequency modulation techniques.
Because recorded magnitudes and playback magnitudes cannot be precisely held, it is impractical to record the entire PWM signal on tape. It is therefore common in recording PWM information, to record only the level transitions, or zero crossings, on magnetic tape. One standard method is to record a narrow pulse at each level transition of the PWM signal. For example, a recorded positive pulse could represent a positive-going level transition of the original PWM signal, while a negative pulse could represent a negative-going transition.
If the position of the transitions could be determined with precision, there would be no difficulty in precisely reconstructing the original PWM signal. There are, however, substantial problems in precisely determining the level transitions of the original PWM input signal.
Ideally, the voltage on the playback head would indicate zero crossings only at the desired level transitions, corresponding to the pulses recorded by the recording head. Also, the slope of voltage versus time is ideally quite steep in the neighborhood of the desired zero crossings, with the slope at each zero crossing having the same magnitude. The recording level is also ideally uniform.
In practice, the foregoing desirable and ideal conditions do not exist. Rather, random noise produces zero crossings on the playback head at other than the desired zero crossings which represent actual PWM level transitions. The slope of voltage versus time at the zero crossings is not so steep as is desired, nor are the slopes of all zero crossings identical. Furthermore, the overall signal amplitude frequently varies as by reason of nonuniformities in the properties of the magnetic tape, and the uniformity of tape speed is also to be considered.
It is the problem of the precise determination of level transitions in PWM signals and the solution to this problem that the present invention is directed.